This invention relates to magnetic resonance imaging (MRI) methods and systems and, more particularly, to the imaging of moving three-dimensional subjects such as myocardial wall.
In many clinical applications, high-resolution MR images of moving three-dimensional (3D) objects must be acquired. For example, it is desirable to acquire images of the myocardial wall, a three-dimensional object that-moves substantially during the cardiac cycle. Generally, a two-dimensional (2D) slice acquisition has been the typical technique to capture a portion of the subject myocardial wall in a 2D imaging plane at some point in the cardiac cycle. However, it may not always possible to capture the same portion at other cardiac phases because the wall region moves in and out of the 2D slice constantly as the heart beats.
MR tagging is a promising technique for non-invasively studying regional heart wall motion both at rest and during stress. With this technique, following the establishment of a number of tags in the myocardium through spatially dependent excitation (usually applied immediately after the R-wave), a sequence of images is acquired at various phases of the cardiac cycle. These images capture the time-varying configuration of the fiducial marks as they deform along with the myocardium and intersect with the imaging plane(s), allowing the myocardial deformation be examined and the motion of individual segments to be tracked. Extensive efforts have been directed towards designing and validating various tagging schemes suitable for quantifying complex myocardial motion. One very efficient scheme, called spatial modulation of magnetization (SPAMM), uses short RF pulses with interleaved gradient pulses to simultaneously generate a set of parallel tags. Some other schemes tag the myocardium with different patterns, e.g., radial tags, which are also suitable for examining the heart geometry.
Use of MR tagging for studying complex heart wall motion generally requires the acquisition of time-resolved volumetric data that capture the 3D motion of the tags. Nevertheless, even on a state-of-the-art MR scanner, the volumetric imaging protocol is too time-consuming to be feasible or of much clinical utility. As a compromise, one typically uses instead a planar imaging protocol, which is imaging a fixed plane (thin slice) in 3D at various phases of the cardiac cycle. With interactive graphic prescription one can specify a plane that contains the myocardial region of interest at a certain cardiac phase. As a consequence of through-plane motion of the heart, however, this approach of focusing on a plane fixed in space is generally limited in its ability to capture the same myocardial region (and embedded tag segments) at other cardiac phases. Thus, the region of interest generally moves constantly in and out of the image plane as the heart beats, and the data acquired at a given cardiac phase only portray the deformation state of a myocardial region (and embedded tag segments) that occurs in that image plane at that phase.
What is needed is an MR imaging method that keeps a moving and deforming region of interest in view, so as to result in an image or series of images that capture the moving and deforming region of interest and any imposed tags over an extended portion of the cardiac cycle.
A method for producing an image of a selected region of interest within a moving and deforming object in a Magnetic Resonance Imaging (MRI) system includes applying at least one integrated preparatory pulse sequence to the region of interest prior to any subsequent imaging sequences. The integrated preparatory pulse sequence is adapted to simultaneously alter longitudinal magnetization of spins located outside of the selected region and to further impose a grid pattern on the selected region.